In recent years, electronic devices including personal computers, televisions, mobile phones, or the like, as representative examples, are developed rapidly and the development progresses with the aim for obtaining the devices with higher density, high output, and light weight. As the performance of the electronic devices is increased, the heat generated per unit area is increased. If the electronic devices are placed in a high temperature environment for a long period of time, an operation becomes unstable and causes a malfunction, a decrease in the performance, or a failure. Thus, a need for dissipating the generated heat more efficiently is increased.
Thermal management is set also with respect to an illumination apparatus using a light emitting diode (LED) as a light source, of which the demand has been drastically increased because the LED has a long life, low electrical power consumption, and a low environmental load as compared with an incandescent lamp or a fluorescent lamp. So far, a metal material or a ceramic material has been mainly used for a member requiring high heat dissipation. However, for decreasing the size of electric-electronic parts, the metal material or the ceramic material suitable has difficulty in decreasing the weight or molding workability and has been being replaced with a resin material.
A thermoplastic resin is easy to mold and has excellent appearance, economical properties, and mechanical strength, in addition to excellent physical and chemical properties. However, since the resin-based material generally has low thermal conductivity, it has been studied to blend a thermal conductive filler in the thermoplastic resin to increase thermal conductivity.
Also, a curable resin is a material which has been widely used for an electrical insulating material, a semiconductor encapsulating material, a fiber reinforced composite material, a coating material, a molding material, and an adhesive material. Among these purposes, in particular, heat dissipation is required in the adhesive, the semiconductor encapsulating material, the electrical insulating material, and the printed circuit board material, and it has been studied to blend a thermal conductive filler in the curable resin to increase thermal conductivity.
As one of the thermal conductive fillers used, aluminum oxide having a size on the order of is used. Aluminum oxide has various crystalline forms such as α, β, γ, δ, and θ, but it is known that aluminum oxide having an α crystalline form has the highest thermal conductivity. However, in general, since aluminum oxide having an α crystalline form has a plate shape or an irregular shape, a problem of increasing viscosity occurs, and it is not possible to incorporate a large amount of the aluminum oxide, even if it is intended to incorporate a large amount of the aluminum oxide into an organic polymer compound in order to obtain high thermal conductivity.
In order to incorporate a large amount of aluminum oxide, in general, as the thermal conductive filler, into the organic polymer compound, spherical aluminum oxide particles are used, PTL 1 discloses a method for manufacturing an epoxy resin composition for semiconductor encapsulation using spherical aluminum oxide particles (spherical alumina), and PTL 2 discloses spherical alumina powders and a resin composition. However, in general, the spherical aluminum oxide particles are aluminum oxide particles having a θ crystalline form or δ crystalline form from the aspect of its manufacturing method, which have low thermal conductivity and, and therefore, a high thermal conductivity comparable to that of aluminum oxide having an α crystalline form cannot be exhibited.
PTL 3 discloses a method for manufacturing an octahedral or higher, that is, polyhedral α-alumina, which is calcined with a fluorine compound, or a fluorine compound and a boron compound; and the alumina can be used as a filler for heat dissipation. However, PTL 3 does not disclose the purity or the thermal conductivity of the obtained polyhedral α-alumina. PTL 4 discloses a resin composition and a rubber composition in which two types of alumina from various types of alumina are used in a particular ratio, and the various types of alumina available in an industrial scale include alumina powders composed of polyhedral primary particles having a particular particle distribution and substantially not having a fracture, alumina by a Bayer's process, electro-melted alumina, and alumina by a hydrolysis method of an organic metal, and that the high thermal conductivity appears to be obtained under the particular blending conditions.
PTL 5 discloses a thermal conductive pressure-sensitive adhesive composed of an acrylic polymer having a high molecular weight as a main polymer and aluminum oxide, in which 95% by weight or more of the aluminum oxide is α-aluminum oxide. PTL 5 discloses that, with respect to the aluminum oxide, a particle shape and essentially has only an α crystal, and the aluminum oxide having a particular inner structure, a particular crystallinity, a particular shape factor, or a particularly regular or irregular appearance is basically not important, and does not define the details of α-aluminum oxide.
In addition, as a thermoplastic resin composition, PTL 6 discloses a resin composition in which α-alumina in a particular amount is blended with a polyphenylene sulfide resin having a particular melt viscosity. However, PTL 6 only discloses that spherical alumina is preferable as the α-alumina, but the details of the α-alumina are not disclosed. Also, PTL 7 discloses a resin composition composed of polyphenyiene sulfide, alumina of which the α crystal particle size is less than 5 μm, and a plate-shaped filler. However, PTL 7 only discloses a ratio of the average particle size of the alumina to the α crystal particle size, but the details of the α-alumina are not disclosed.
PTL 8 discloses a resin composition including a polyphenyiene sulfide resin and two types of a alumina having a particular size of α crystal particle size, in which one or both of the two types of α alumina are a alumina pre-treated with a coupling agent. However, PTL 8 only discloses the α crystal particle size, but the details of the α alumina are not disclosed.
In any of a curable resin type and a thermoplastic resin type, it is necessary to incorporate a large amount of the aluminum oxide having a size on the order of μm as a filler in order to obtain a resin composition having high thermal conductivity. In order to incorporate a large amount of the aluminum oxide, in general, spherical alumina is used, but since this is the α-alumina including many θ or δ crystals, a sufficient thermal conductivity is not obtained.
In addition, in general, the thermal conductivity of aluminum oxide is greatly affected by its purity, those skilled in the art considers that a filler having an aluminum oxide component in a low amount does not exhibit a high thermal conductivity, and it is necessary to highly purify a raw material to be used and/or a product in order to obtain a highly pure aluminum oxide. Therefore, there is a problem in that the cost of the filler obtained by having gone through this refinement step is increased, or the like.